The present invention relates to photomultiplier tubes and particularly to an improved focusing structure which alters the electron-optical field in the vicinity of the secondary emitting surface of the primary dynode.
Electron emissive electrodes are used in photomultiplier tubes to emit an electron in response to each impinging photon or a plurality of secondary electrons for each impinging primary electron. The primary electrons can be photoelectrons from a photocathode or secondary electrons from another dynode. The problem that has been encountered in the construction of phototubes has been to efficiently collect electrons emitted from one stage of an electron multiplier by another stage. In particular, the problem has been to maximize the collection of electrons at the input stage of the electron multiplier, i.e., photoelectrons from a photocathode to the first dynode of an electron multiplier. An increase in the efficiency of collection of electrons at the input stage increases the signal-to-noise ratio.
A first dynode having a large collection area is disclosed in U.S. Pat. No. 4,112,325 to R. D. Faulkner, issued Sept. 5, 1978. The dynode is generally teacup shaped having a substantially circular top opening facing the photocathode and a flat base. A generally curved side wall connects the base to a circular rim around the periphery of the top opening. The side wall includes a flat region having an aperture through which secondary electrons exit the dynode. The region of the dynode opposite the exit aperture including a portion of the lower sidewall is generally referred to as the "heel" region. In the operation of the teacup dynode, it has been determined that secondary electrons emitted from the "heel" region of the dynode are not effectively focused onto the secondary emissive areas of the second dynode because the strong negative electrostatic field from the photocathode suppresses the emission of secondary electrons from the "heel" region of the dynode and causes those secondary electrons which are emitted from the "heel" to be returned to the teacup dynode.
In many applications such as scintillation counting, for example, it is required that the output of a photomultiplier be linear with light input. Since the light energy of scintillations is directly proportioned to the gamma-ray energy over a certain range, an electrical pulse obtained from a photomultiplier tube is a direct measure of the gamma-ray energy. Consequently, an important requirement of photomultipliers used in scintillation counting is the ability to discriminate between pulses of various height. The parameter indicating the ability of a tube to perform this discrimination is called pulse-height resolution. The pulse-height resolution of photomultiplier tubes having a preliminary teacup dynode may be improved by effectively utilizing all the secondary electrons emitted by the active area of the first dynode and focusing these secondary electrons onto the active area of the second dynode. The active areas or surfaces comprise a surface region of the dynodes from which secondary electrons may be generated, and from which the secondary electrons may then be properly accelerated as an electron stream to subsequent elements, in ordered sequence, for ultimate collection by the anode.